The present invention relates to a device for bringing about coincidence between the axis of a measuring probe or head and the normal to the surface of a part to be controlled. It is more particularly applicable to measurements of thicknesses of thin layers or to the determination of scatter profiles by back-scattering of electrons.
The aforementioned thin layers can in particular be of metal and are, for example, deposited on a substrate formed from metallic or non-metallic materials. For example, it may be desired to measure the thickness of a thin copper layer deposited on a resin. The thickness of the thin layer or layers deposited on the substrate may obviously be variable, i.e. non-uniform.
Moreover, the surface of the part to be controlled may have different shapes. It may be a surface having at least one axis of revolution, such as a spherical surface. The present invention is more particularly applicable to measuring the thickness of thin layers deposited on the surface of a hemispherical member or a body of revolution, such as a member shaped like a semi-ellipsoid of revolution.
For example, apparatuses for measuring the thickness of thin layers by electron back-scattering or .beta. back-scattering are known, which comprise a measuring probe or measuring head able to emit electrons onto a part comprising one or more thin layers deposited on a substrate. The electrons which are back-scattered by these thin layers are then detected by detection means placed in the probe and making it possible to calculate a "counting rate" corresponding to the number of back-scattered electrons detected per unit of time. This counting rate is characteristic of the thickness of the layers, the chemical nature of the layers and the energy of the electrons used, and since the two latter factors are known, the counting rate can be used for measurements of layer thicknesses that vary between a few microns to several hundred microns.
It is known that the two main sources of errors when measuring thin layer thicknesses by .beta. back-scattering are a lack of perpendicularity of the probe relative to the part to be controlled that carries the layer or layers under investigation and the distance between the probe and the part to be controlled.
It has been found that the error in the thickness measurement of a layer increases rapidly with the lack of perpendicularity or the "angular separation" between probe and part. Moreover, the sensitivity of the measuring apparatus decreases as the distance between the probe and the part increases. The resulting errors are explained by the lack of detection of all the back-scattered electrons from the analysed area of the part to be controlled.